The present invention relates to digitizers and, in particular, it concerns devices for use with conventional presentation boards and pens for digitizing lines drawn manually.
It is known to use various techniques for determining the position of a writing implement or stylus on a flat surface. U.S. Pat. No. 4,564,928 to Glenn et al., U.S. Pat. No. 4,886,943 to Suzuki et al., U.S. Pat. No. 4,910,363 and U.S. Pat. No. 5,073,685 to Kobayashi et al. and U.S. Pat. No. 5,097,102 to Yoshimura et al. all disclose systems in which a vibrating element associated with a pen transmits vibrations through the material of a board. The vibrations are detected by transducers attached to the board and the position of the pen is calculated from the transmission time of the vibrations through the board. These systems inherently function exclusively when the pen is in contract with the board such that vibrations are transferred to the board. As a result, no special mechanism is required to distinguish writing from non-writing pen movements.
These systems are generally inaccurate due to non-uniform transmission times through the board. In fact, they typically require highly specialized board structures which renders them expensive and inconvenient.
An alternative approach is the use of air-borne ultrasound signals. Examples of such systems are described in U.S. Pat. No. 4,777,329 to Mallicoat, U.S. Pat. No. 4,814,552 to Stefik et al., U.S. Pat. No. 4,506,354 to Hansen and U.S. Pat. No. 1,758,691 to De Bruyne. These systems employ various combinations of ultrasound transmitters and receivers arranged at two points fixed relative to a board and on a movable writing implement. The position of the movable implement is then derived by triangulation. The systems typically require an additional hard-wired or electromagnetic link between the movable implement and a base unit to provide timing information for time-of-flight ultrasound calculations. An additional switch is also required to identify when the movable element is in contact with the board.
These systems are typically limited to relatively small boards. This is be because of signal to noise ration (SNR) limitations. The volume of ultrasound used cannot be very high without causing bothersome accompanying whistling noises. Additionally, in a wireless system, power considerations severely limit the transmitted volume. In order to generate reliable position information, the transmitter-to-receiver distance must therefore be kept small. Attempts to use different sets of receivers for different regions of a large board generally result in discontinuities when the movable element travels from one region to another.
Another shortcoming of these systems is their inability to reproduce rapid interrupted pen strokes such as performed when drawing a dashed line. Typically, the transmitter or receiver element in the pen turns off when the pen is inactive and is re-activated each time the pen comes in contact with the board. The system then takes a fraction of a second to resynchronize before it responds correctly. In the case of short strokes, the length of the operative stroke may be comparable with the response time of the system, thereby giving very poor results.
An additional problem of the airborne ultrasound digitizer systems is that the ultrasound transmitter or receiver element is mounted asymmetrically to the side of the drawing implement. As a result, the measured position is offset from the true drawing position in a direction which changes with rotation of the drawing implement. This may result in discontinuities and illegible writing in the digitized images when the drawing implement position is changed between strokes.
Finally, conventional presentation board digitizer systems are typically limited to use with specially produced writing implements. This renders them expensive since pens have a very limited lifetime. Even where the ink cartridge is separately replaceable, the components used must be of a very specific design to be compatible.
There is therefore a need for a reliable, low-cost, digitizer system which may be used with conventional presentation boards of all sizes for determining accurately the position of a drawing implement on the board. It would also be advantageous to have a transmitter device for use with presentation board which can be used with a wide range of conventional writing implements.
The present invention is of presentation board digitizer systems for use with presentation boards of all sizes which allow accurate reproduction of short pen strokes and which may be used with conventional writing implements.
According to the teachings of the present invention there is provided, an ultrasound receiver assembly for use in a presentation board digitizer system, the receiver assembly comprising: (a) a first ultrasound receiver located adjacent to the surface of the presentation board; and (b) a second ultrasound receiver displaced from the first ultrasound receiver in a direction substantially perpendicular to the surface of the presentation board, the first and second ultrasound receivers being connected so as to generate a total output signal corresponding to the instantaneous sum of the ultrasound signals received at each of the first and second ultrasound receivers such that the receiver assembly is most sensitive to ultrasound signals incident from a plane substantially adjacent to the presentation board.
According to a further feature of the present invention, the first and second receivers are connected in series.
In the context of an ultrasound based digitizing system for identifying the position of an ultrasound transmitter associated with an element movable relative to a surface, the system having at least three spaced apart ultrasound receivers associated with the surface, there is also provided according to the teachings of the present invention, a method of analyzing outputs from the ultrasound receivers comprising the step of identifying as a current position a weighted centroid of at least a first calculated position derived from the outputs of a first pair of the receivers and a second calculated position derived from the outputs of a second pair of the receivers, wherein the weighting varies as a continuous function of approximate position relative to the ultrasound receivers.
According to a further feature of the present invention, the ultrasound receivers are substantially collinear, and the weighting varies linearly with distance in the direction of alignment of the ultrasound receivers over at least a given switch-over zone.
According to a further feature of the present invention, the weighted centroid approximates to the first calculated value when the movable element is within a first given region of the surface.
There is also provided according to the teachings of the present invention, a presentation board digitizer system for digitizing operative strokes of a drawing implement carrying an ultrasound transmitter against the board, the system comprising: (a) at least two ultrasound receivers mounted relative to the board for receiving air-borne ultrasound signals; (b) a transducer associated with the board so as to detect vibrations from the transmitter conducted through the board; and (c) a processor responisive to outputs from the at least two ultrasound receivers to calculate a current position of the transmitter, the processor being additionally responsive to an output from the transducer to identify contact between the drawings implement and the board, thereby identifying operative strokes of the drawing implement.
There is also provided according to the teachings of the present invention, a transmitter device for use with a system for digitizing operative strokes of a hand-held drawing implement, the drawing implement having a body and an operative tip, the transmitter device comprising: (a) a housing having a substantially cylindrical opening terminating at a first end in an annular wedge surface with a central bore, the housing receiving a portion of the body of the drawing implement with its operative tip extending from the central bore; (b) a retainer attachable to a second end of the opening to retain the drawing implement within the housing, the retainer having a spring element for biasing the drawing implement towards the annular wedge surface; and ĉ a transmitter mounted relative to the housing proximal to the central bore.
According to a further feature of the present invention, the housing further includes: (a) a microswitch actuated by changes in pressure exerted on the annular wedge surface so as to be responsive to a force exerted on the operative top of the drawing implement towards the housing; and (b) electronic circuitry responsive to the microswitch indicates a force exerted on the operative tip of the drawing implemented towards the housing.
According to a further feature of the present invention, the electronic circuitry operates the transmitter for a given time interval after the microswitch ceases to indicate a force exerted on the outer housing towards the operative tip of the drawing implement.
According to a further feature of the present invention, the given time interval is at least about half a second.
According to a further feature of the present invention, the transmitter transmits continuously, and the electronic circuitry is responsive to the microswitch to change a signal transmitted by the transmitter while the microswitch indicates the force exerted on the operative tip of the drawing implement towards the housing.
According to a further feature of the present invention, the transmitter is an ultrasound transducer.
According to a further feature of the present invention, there are also provided elements of an electromagnetic communciations link, the elements being associated with the electronic circuitry.
There is also provided according to the teachings of the present invention, an ultrasound transmitter device for use with a system for digitizing the position of a hand-held drawing implement, the drawing implement having a central axis and an operative tip, the transmitter device comprising a substantially cylindrical piezoelectric transmitter element positioned coaxially with the drawing implement so as to circumscribe a part of the drawing implement proximal to the operative tip.
There is also provided according to the teachings of the present invention, an eraser device for use with a presentation board digitizer system, the eraser device comprising: (a) a handle; (b) an eraser element having a substantially flat eraser surface; and (c) a pivot joint connecting between the handle and the eraser element, the pivot joint having two degrees of rotational freedom such that, in use, the eraser element assumes an orientation with the eraser surface parallel to the presentation board surface substantially independent of the orientation at which the handle is held.